The present invention relates to a method of controlling the operating temperature of a hydraulic operating medium of a drive unit of a vehicle by controlling the volume flow of the operating medium through a heat exchanger and to a control device for performing the method.
DE 195 12 783 A1 discloses a method and a device for controlling the transmission oil temperature in motor vehicles, including a cooling device for cooling the transmission oil, a heating device for heating the transmission oil and a valve which directs the transmission oil through the heating device in a first operating state and through the cooling device in a second operating state. According to the patent, the known method is to be improved in such a manner, that the transmission oil temperature is controlled as a function of vehicle parameters which have an effect on the transmission oil temperature. For this purpose, the first operating state is established in a warm-up phase and, after the warm-up phase, the transmission temperature is regulated to a predetermined desired value in a third operating state by a mixed operation of the first and second operating states. In this known method, the selected range of the operating temperature for setting the latter to the desired value accordingly does not begin until after the warm-up phase. Both operating states are based on the operation of the internal combustion engine, particularly after a cold start, and also on the transmission, particularly on the reaching of a minimum transmission oil temperature. Control may also be based on the idling mode of the internal combustion engine, since, during a prolonged idling, the transmission oil temperature may drop below the predetermined desired value. Also, in this case, the first operating state of the electrically controllable valve, in which the valve directs the volume flow of oil through the heating device, is considered to be advantageous. The above-mentioned valve, the functioning of which is disclosed by DE 43 24 178 A1, has a valve actuating unit in the form of a temperature-responsive expandable element which is encased by a heating coil and is arranged in a valve chamber through which the transmission oil flows. The temperature responsive element and is exposed to by the transmission oil. The transmission oil can be heated via an electric power supply line, which is connected to an electronic control unit. The control unit has a plurality of inputs by means of which, for example, the following signals can be passed on as information to the control unit: the transmission oil temperature, the output rotational speed of the transmission, the rotational speed of the internal combustion engine or the difference between these rotational speeds, a load signal of the internal combustion engine, the selected gear, the activated gear change program for an automatic changing of gears, the switching-on signal of the converter lockup clutch and/or the ascertained type of driver in the case of an adaptive gear change program. The parameters, which are merely specified in the form of an exemplary selection, are intended to be used for activating the electrically controllable valve for setting the volume flow of oil in one or both heat exchangers (heating device and cooling device).
This known method and the associated device for carrying out the known method are costly and complicated because three operating states have to be controlled. A heating device, a cooling device and special functions of the control valve are required, which have to be activated by the temperature-sensitive expandable element and which furthermore must be heated for the activation. Moreover, it is not expedient in many applications to control the oil temperature exclusively by a mixed heating and cooling operation.
DE 199 02 615 A1 discloses a hydraulic system, for an infinitely variable automatic transmission of a motor vehicle, having a variator, which has a first pair of conical discs arranged on the drive side and a second pair of conical discs arranged on the output side. A belt element extends around the first and second pairs of conical discs. Each pair has a primary disc, which can be displaced in a controlled manner in the axial direction via a hydraulic control unit, which has electromagnetic actuators and hydraulic valves. The actuators are controlled by an electronic control unit. A cooler for the oil is arranged in the lubricating/cooling circuit of the hydraulic system and a line leads therefrom to the oil sump. It has been established that, during full load operation, in such transmissions the available amount of oil flowing through the cooler by way of fixed orifices is not sufficient in order to completely dissipate the heat via the cooler. Furthermore, the amount of oil flowing through the cooler may be restricted by the fact that, with regard to the cooling-induced discharge of oil from the region of the converter clutch, the power transmission capability of the latter still has to be sufficient for the customary driving maneuvers taking into consideration the maximum requirement in terms of volume for adjusting the primary disc and the secondary disc of the variator. In order also to ensure sufficient cooling of the oil while ensuring the power transmission capability of the converter clutch in the case of infinitely variable automatic transmissions of this type, which are provided with suction-restricted radial piston pumps, the hydraulic system includes a orifice arranged, as shown, in the direction of flow, downstream of the cooler and on the secondary side of the variator, in the line between the cooler and oil sump.
The orifice cross-section can be controlled between a first position, in which it is closed, and a second position, in which it is completely open, as a function of the rotational speed of the drive motor of the vehicle and of the vehicle traveling speed. The orifice is configured in such a manner that it does not begin to open the passage until a predetermined traveling speed is reached. If, at full load operation and maximum speed, the variator is operated at a single operating point, namely the top transmission ratio, then a highly dynamic adjustment of the variator will no longer take place and the amount of oil flowing through the cooler will be increased by the fact that, from a certain traveling speed, the orifice will permit the drainage of oil into the oil sump. The orifice opens from this traveling speed on to a higher traveling speed as a function of the increasing rotational speed until the orifice is completely open.
For example, in a transmission of a motor vehicle, it is expedient, or required, for the transmission oil to be within a certain temperature range. In order to fulfill the requirement, at least one heat exchanger is provided for cooling or for heating the transmission oil. The oil flows through the heat exchanger at one side thereof, and a secondary flow medium (liquid or gas as coolant), flows through the other side. As a result, the secondary flow medium can absorb heat from the transmission oil or deliver heat to the transmission oil. The heat transfer in the heat exchanger is dependent on a plurality of physical state variables. One important physical state variable for the heat transfer is the volume flow of transmission oil per unit time through the heat exchanger. With increasing volume flow of transmission oil, a higher heat transfer can take place within the heat exchanger.
It is the object of the present inventions to provide a predetermined desired flow of hydraulic operating medium, for example of transmission oil, through a heat exchanger as a function of certain parameters with only a small functional and structural outlay.